Metal ion laser having an auxiliary gas

ABSTRACT

There are disclosed cadmium ion lasers in which gettering of helium by cadmium deposited on walls near the discharge is counteracted by maintaining the coolest region frequented by cadmium ions above about 100* C, although preferably about 150* C, and away from the discharge. One embodiment employs a tube having an enlarged region or sidearm to remove the coolest region from the discharge. A preferred embodiment disposes the cathode in a housing of larger diameter than the diameter of the tube around the discharge, the cathode being at a position in the housing establishing a variation in wall temperature to compel the spent cadmium to be deposited behind the cathode with respect to the discharge.

United States Patent 1151 3,663,892

Klein et al. 1 51 May 16, 1972 [54] METAL ION LASER HAVING AN Fendly,Jr. et al., RCA Review, 30, Sept. 1969, pp. 422 42s.

AUXILIARY GAS Silfuast et al., Applied Physics Letters, 13, 1968, pp.169+ [72] Inventors: Marvin Bertrand Klein, Long Beach; 7 primaryExaminer wimam Lsikes Thomas Patrick Sosnowski, Colts Neck,AssismmEmminer R lwebster both of AttorneyR. J. Guenther and Arthur J.Torsiglieri 73 Assi nee: Bell Tele hone Laboratories lncor orated l gMurray rim, NJ. p [571 ABSTRACT [22] Filed: June 15, 1970 There aredisclosed cadmium ion lasers in which gettering of helium by cadmiumdeposited on walls near the discharge is PP N05 46,097 counteracted bymaintaining the coolest region frequented by cadmium ions above about100 C, although preferably about 52 us. (:1 ..331/94.5 313/223 313/225and the discharge- One emimdimem 1 Int. (:1 ..l-l0 1s 1/06 1 1015 3/09Phys 3 tube haYmg enlarge? sidearm [58] Field Search 331/945 313/223 thecoolest region from the discharge. A preferred embodiment disposes thecathode in a housing oflarger diameter than [56] References cued thediameter of the tube around the discharge, the cathode being at aposition in the housing establishing a variation in OTHER PUBLICATIONSwall temperature to compel the spent cadmium to be deposited behind thecathode with respect to the discharge. Goldsborough, Applied PhysicsLetters, 15, (6), Sept.

1969, pp. 159- 161. 10 Claims, 2 Drawing Figures CATHODE 47 HEATING I 3837 souacs 37 ANODE M +1 11 fi l M Cd CONDENSATION ANODE 21 CATHODEBEHIND CATHODE g 39 4 36 i I 0 0 I I 43 32 Cd P44 34 22 HEATING 45SOURCE 45 BACKGROUND OF THE INVENTION This invention relates to gaslasers, particularly those in which one of the constituent gases can beoccluded or gettered by condensation of another constituent on the tubewalls.

One of the most promising gaseous ion lasers described in the literatureof the laser art is the cadmium ion laser that employs helium as anauxiliary gas. This laser is of interest because it can operate both at441.6 nm (nanometers) in the blue region of the spectrum and at 325.0 nmin the ultraviolet region of the spectrum and can produce acontinuous-wave or pulsed output with modest discharge currents. Thesefrequencies are attractive for many uses, such as optical memories. 7

One of the most significant problems encountered when operating sealedoff helium-cadmium lasers is in the rapid loss of helium from thegaseous volume in and around the discharge. In the most severe cases,several torr (l torr equals 1 millimeter of mercury) pressure of thehelium may be lost after only a few hours of tube operation.

This loss of helium is much too rapid to be explained by diffusionthrough the glass walls of the discharge tube. A more reasonableexplanation may be inferred from our observations that, after suchhelium loss, large quantities of helium could be released into the tubevolume by heating areas of the tube where cadmium had condensed. Incontrast, the heating of other areas of the tube had no effect on thehelium partial pressure.

SUMMARY OF THE INVENTION Accordingly, our invention is based on ourdiscovery that helium is lost or gettered in areas where cadmium vaporis condensing and, further, on our discovery that the net loss istemperature dependent and also dependent upon theproximity of thedischarge to the region of condensation. Our observations show that theamount of helium that can be occluded or gettered by the condensingcadmium is inversely related, though not inversely proportional, to thedistance from the discharge to the region of condensation. There existsa separation from the active discharge region at which the ability ofcondensing cadmium to occlude or getter helium can be made negligible.At lesser separations the gettering is small if the temperature of theregion of condensation is high enough, il- Iustratively above about 150C.

One embodiment of a gas laser according to our invention provides aregion of the tube for cadmium vapor condensation relatively removedfrom the discharge, or enlarged to reduce the intensity of the dischargethere, and maintains that region above about 150 C but lower intemperature than other regions of the tube accessible to the cadmiumvapor.

Specifically, in the first embodiment of our invention, the tube wasflared to a substantially increased diameter at an axial position nearthe cathode along the discharge path. For the typical dischargeconditions, the desired temperature of the flared region was obtained bya simple wrap of aluminum foil to provide some thermal insulation.

In a preferred embodiment of our invention, the cathode is disposed at aposition in its housing to establish along the housing portion in frontof the cathode a decrease in wall temperature extending to the regionbehind the cathode, where the cadmium vapor then condenses. Typically,the cathode is moved closer to the small-diameter portion of the housingthan in the prior art; and a larger empty volume is left behind thecathode.

The latter embodiment of the invention is advantageously employed in adouble-ended tube having a centrally disposed cathode.

Nevertheless, reversible single-ended tubes can be used according to ourinvention. In such tubes, the condensation region is similar to thecadmium reservoir and occupies a position near the cathode symmetricalto the position of the reservoir near the anode. When the roles of anodeand cathode are interchanged, the roles of reservoir and condensationregions are interchanged. The modified use of such reversible tubesaccording to our invention includes simultaneous heating of both thereservoir and condensation region to reduce the temperature difierencetherebetween, in view of the closeness of the condensation region to theactive discharge. The condensation region temperature is preferablymaintained above 150 BRIEF DESCRIPTION OF THE DRAWING Other features andadvantages of our invention will become apparent from the followingdetailed description, taken together with the drawing, in which:

FIG. 1 is a partially pictorial and partially schematic illustration ofthe early embodiment of our invention; and

FIG. 2 is a partially pictorial and partially schematic illustration ofthe preferred embodiment of our invention.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS In the embodiment of FIG. 1, theillustrated cadmium ion laser can be operated to obtain a useful outputat 441.6 nm (nanometers 1 X 10 meters) in the blue region of thespectrum or at 325.0 nm in the ultraviolet region of the spectrum, orboth, by appropriate choice of the reflectors 21 and 22 comprising theoptical resonator. They may be multiple-layer dielectric coatedreflectors or internal reflection prisms (not shown).

The gas mixture of cadmium vapor with about 5 torr of helium as anauxiliary gas is contained in the tube 12 of high-temperature glass andof 3 millimeter internal diameter in the central portion. Tube 12 hasquartz end windows 13 and 14 with antiparallel Brewster-angleorientation.

A discharge is established through the gas mixture by the conventionalpumping circuit including, in series, anode l5, ballast resistor 18,battery 17 and cathode l6 heated by conventional means including theheating current source 27.

Tube 12 is flared at the ends to form regions 19 and 20 0f about 8millimeters inside diameter. Just beyond flared region 20 in a directionaway from anode 15 is located the sidearm or reservoir 23 containingcadmium of the desired isotope mixture. The reservoir 23 and the cadmiumare heated by a heating coil 24 energized by the heating current source25 to vaporize the cadmium controllably, although the cadmium could alsobe vaporized by a discharge of sufficient intensity if non-vaporizedcadmium is relatively close to the discharge.

To prevent gettering of helium by cadmium condensing in region 19, thatregion was loosely wrapped in our early experiments with tinfoil tomaintain its temperature illustratively about C. The tinfoil is oneexample of insulation 26 which may be provided for a portion of region19 appreciably removed from end window 13. We have found that cadmiumvapor condenses in region 19 before it can reach window 13. For longestoperating life of the laser, insulation 26 is modified to maintain thetemperature of the insulated region at about 150 C, as will beunderstood from the following description of operation.

In operation, the discharge, e.g., at a current level of milliamperes,ionizes the cadmium vaporized from reservoir 23. The electric fieldgradient along the discharge transports the positive cadmium ions by aneffect called cataphoresis toward cathode l6. Metastable helium atomsare formed in the discharge and facilitate the establishment of apopulation inversion in the ionized cadmium, apparently by a collisionalenergy transfer in which appropriate elevated energy levels of thecadmium ion are populated and in which excess metastable energy isimparted to free electrons. The stimulated emission of radiation now canresult at the resonated wavelength or at the wavelength of an inputsignal.

With the decrease in temperature from reservoir 23 to region l9, cadmiumcondenses only in the region 19, which is the coolest region accessibleto cadmium vapor. At this temperature and slightly removed from theactive discharge, or at least at a less intense region of the discharge,the condensed cadmium in region 19 getters much less helium than in theprior art. The active discharge has a lower current density, and thus alower intensity, in this region because of the increased cross-sectionalarea of the tube.

It would be desirable to condense the transported cadmium vapor fartherfrom the active discharge than is feasible in the embodiment of FIG. 1,as our later experiments have shown that the helium loss rate waslargest in regions where the discharge in adjacent tube volumes wasstrongest.

A preferred way to satisfy all of the criteria set out above is toinduce condensation of cadmium only on the walls of the cathode housingbehind the cathode. The shaping of the walls around the cathode can thenmaintain the desired temperature distribution having its lowest valuebehind the cathode. The discharge is nonexistent there.

Such a modified embodiment is shown in the double-ended tube arrangementof FIG. 2, in which components similar to those of the embodiment ofFIG. 1 are numbered with numbers 20 digits higher than the correspondingcomponents in FIG. 1. Similar components in the symmetrical halves ofthe arrangement are numbered the same except for primed numbers in theright-hand half of the embodiment of FIG. 2.

The principal difference from the embodiment of FIG. 1 resides in thelack of any significant flare in tube 32 in the vicinity of the housing39 for cathode 36. Indeed, the entry portion 40 of the housing can haveapproximately the same diameter as tube 32. The cathode 36 is disposedsufficiently close to the walls of the portion 41 of cathode housing 39to heat portion 41 so that cadmium cannot condense there. The lowesttemperature of the housing 39 occurs at its wall 42 farthest (e.g.,about 7 centimeters but at least centimeters) behind the cathode awayfrom the active discharge. Cataphoresis transports cadmium to housing39; and the cadmium condenses on the wall 42.

The operation of the embodiment of FIG. 2 is substantially similar tothat of the embodiment of FIG. 1 with the exception that the helium losscan be almost totally avoided with the lack of discharge in theimmediate vicinity.

The reason for this fact is not well understood; but the variousmechanisms of gettering may include not only the burying of helium atomsby subsequently condensed cadmium atoms when the helium atoms are in thevicinity of or adhering to the walls (occlusion of the auxiliary gas),but also may include tunneling or diffusion of accelerated helium ionsinto cadmium layers already present. The latter explanation wouldprovide one reason for the strong dependence of the helium loss on theintensity of the electrical discharge in the surrounding volume.Nevertheless, we do not wish our invention to be limited by thisexplanation. In any event, all of the possible mechanisms may be termedgettering.

We claim:

I. A laser comprising a tube supplied with helium and having a reservoircontaining cadmium, a cathode and anode disposed to supply adirect-current discharge through said tube in said helium to drive vaporof said cadmium from said reservoir toward said cathode and to invertthe population of a radiative transition in said vapor, said reservoirbeing disposed nearer to said anode than to said cathode, meanscomprising an enlarged portion of the tube nearer to said cathode thanto said anode and substantially removed from the electric field of thedischarge for providing a condensation region for the driven cadmium ata temperature at least as low as a condensation temperature for saidcadmium and above about C whereby gettering of the helium by condensedcadmium is inhibited, and means aligned with the axis of the tube forstimulating the emission of coherent radiation from said vapor.

2. A laser according to claim 1 in which the means for providing thecondensation region of the tube in the aforesaid temperature rangecomprises means for disposing a portion of the tube walls in thevicinity of the cathode at a greater distance from the axis of thedischarge than in a central portion of the tube, and means for thermallyinsulating said portion in the vicinity of said cathode.

3. A laser according to claim 1 in which the providing means formaintaining the condensation region at temperatures above about C.

4. A laser comprising a tube supplied with helium and having a reservoircontaining cadmium, a cathode and anode disposed to supply adirect-current discharge through said tube in said helium to drive vaporof said cadmium from said reservoir toward said cathode and to invertthe population of a radiative transition in said vapor, means forming aportion of said tube on the opposite side of said cathode with respectto the path of said discharge and substantially removed from theelectric field of the discharge for providing a condensation region forthe driven cadmium at the lowest temperature in the cathode housingwhereby gettering of the helium by condensed cadmium is inhibited, andmeans aligned with the axis of the tube for stimulating the emission ofcoherent radiation from said vapor.

5. A laser according to claim 4 in which the tube is shaped to preventcondensation of the cadmium between the reservoir and the condensationregion.

6. A laser according to claim 4 in which the means forming a portion ofthe tube behind the cathode is separated from the cathode by at leastabout 5 centimeters.

7. A laser comprising a tube supplied with an auxiliary gas and having areservoir containing a vaporizable material supplying the active gas, acathode and anode disposed to supply a direct-current discharge throughsaid tube in said auxiliary gas to drive vapor of said active gas fromsaid reservoir toward said cathode and to invert the population of aradiative transition in said vapor, means forming a portion of said tubeon the opposite side of said cathode with respect to the path of saiddischarge and substantially removed from the electric field of saiddischarge for providing a condensation region for said vapor at thelowest temperature in the cathode housing whereby gettering of thehelium by condensed cadmium is inhibited, and means aligned with theaxis of the tube for stimulating the emission of coherent radiation fromsaid vapor.

8. A laser according to claim 7 in which the means forming a portion ofthe tube behind the cathode is separated from the cathode by at leastabout 5 centimeters.

9. A laser according to claim 8 in which the vaporizable material iscadmium and the means forming a portion of the tube behind the cathodeis separated from the cathode by about 7 centimeters.

10. A laser according to claim 7 in which the tube is symmetrical aboutthe cathode sidearm.

* l I! l

2. A laser according to claim 1 in which the means for providing thecondensation region of the tube in the aforesaid temperature rangecomprises means for disposing a portion of the tube walls in thevicinity of the cathode at a greater distance from the axis of thedischarge than in a central portion of the tube, and means for thermallyinsulating said portion in the vicinity of said cathode.
 3. A laseraccording to claim 1 in which the providing means for maintaining thecondensation region at temperatures above about 150* C.
 4. A lasercomprising a tube supplied with helium and having a reservoir containingcadmium, a cathode and anode disposed to supply a direct-currentdischarge through said tube in said helium to drive vapor of saidcadmium from said reservoir toward said cathode and to invert thepopulation of a radiative transition in said vapor, means forming aportion of said tube on the opposite side of said cathode with respectto the path of said discharge and substantially removed from theelectric field of the discharge for providing a condensation region forthe driven cadmium at the lowest temperature in the cathode housingwhereby gettering of the helium by condensed cadmium is inhibited, andmeans aligned with the axis of the tube for stimulating the emission ofcoherent radiation from said vapor.
 5. A laser according to claim 4 inwhich the tube is shaped to prevent condensation of the cadmium betweenthe reservoir and the condensation region.
 6. A laser according to claim4 in which the means forming a portion of the tube behind the cathode isseparated from the cathode by at least about 5 centimeters.
 7. A lasercomprising a tube supplied with an auxiliary gas and having a reservoircontaining a vaporizable material supplying the active gas, a cathodeand anode disposed to supply a direct-current discharge through saidtube in said auxiliary gas to drive vapor of said active gas from saidreservoir toward said cathode and to invert the population of aradiative transition in said vapor, means forming a portion of said tubeon the opposite side of said cathode with respect to the path of saiddischarge and substantially removed from the electric field of saiddischarge for providing a condensation region for said vapor at thelowest temperature in the cathode housing whereby gettering of thehelium by condensed cadmium is inhibited, and means aligned with theaxis of the tube for stimulating the emission of coherent radiation fromsaid vapor.
 8. A laser according to claim 7 in which the meaNs forming aportion of the tube behind the cathode is separated from the cathode byat least about 5 centimeters.
 9. A laser according to claim 8 in whichthe vaporizable material is cadmium and the means forming a portion ofthe tube behind the cathode is separated from the cathode by about 7centimeters.
 10. A laser according to claim 7 in which the tube issymmetrical about the cathode sidearm.